The present invention generally pertains to phase accumulators, such as are used in direct digital frequency synthesizers, and is particularly directed to dithering the phase at which a coarse-component accumulator is incremented in response to accumulation of the fine phase components.
A direct digital frequency synthesizer generates an analog waveform of a predetermined frequency from accumulated digital frequency words, which, as accumulated, represent the phase of a cyclic waveform, such as a sine wave of said predetermined frequency. A typical prior art direct digital frequency synthesizer includes a phase accumulator for accumulating the digital frequency words and a phase-to-magnitude converter for converting the accumulated phase value into an analog waveform of the predetermined frequency. The phase-to-magnitude converter converts the phase value accumulated in the phase accumulator into an analog signal magnitude for the phase angle of the cyclic waveform that is represented by the phase value in the phase accumulator.
The phase accumulator has a length of m bits and is driven at a clock rate f.sub.c. At each clock pulse, a frequency word having a length of k bits is added to the present value in the phase accumulator. The value in the phase accumulator increases at this rate until it overflows losing all bits that exceed 2.sup.m -l. The phase value in the phase accumulator at any time represents the instantaneous phase angle of the cyclic waveform over a range of 2.pi. radians.
There are 2.sup.m different phase values; and the actual output frequency is g times f.sub.c /2.sup.m, where g can range from one to 2.sup.m. The frequency resolution is also equal to f.sub.c /2.sup.m, which means that the resolution requirements can generally be met by trading off f.sub.c and m.
The phase accumulator performs a phase calculation once each clock cycle by adding the frequency word, which is proportional to the desired output frequency, to the contents of the phase accumulator.
It is not practically feasible (or necessary) to retain every bit from the phase accumulator for use in a frequency synthesizer; so the phase accumulator is divided into a coarse-component accumulator and a fine-component accumulator. The full m bits are partitioned in c bits in the coarse-component accumulator and f bits in the fine-component accumulator. Only the c bits of the coarse-component accumulator are used to determine the phase value for one cycle of phase accumulator output, whereby phase resolution is limited to 2.pi./2.sup.c radians. Both the coarse-component accumulator and the fine-component accumulator are clocked to run at a frequency of f.sub.c. The minimum frequency that the coarse-component accumulator can provide is f.sub.c /2.sup.c Hz. The fine-component accumulator provides finer frequency resolution by periodically adding a carry-in to the coarse-component accumulator's LSB of 2.pi./2.sup.c radian. Given that the fine-component accumulator consists of f bits and also runs at f.sub.c, the least carry-in rate it can provide is f.sub.c /2.sup.f. As each coarse LSB is worth 2.pi./2.sup.f. As each coarse LSB is worth 2.pi./2.sup.c radians, the minimum average frequency added by the fine-component accumulator is 2.pi./2.sup.c times f.sub.c /2.sup.f radians/second or f.sub.c /2.sup.c+f Hz. Since m=c+f, the overall resolution, as well as minimum step size remains unchanged by the separation.
Only the bits from the coarse-component accumulator (the c bits) are used to represent the necessary phase information with a resolution of 2.pi./2.sup.c radian. As the remaining fine-component accumulator bits (the f bits) of the phase accumulator are ignored, the output phase function generally has a phase error with respect to the total phase function contained in the phase accumulator. The phase error is slightly periodic in time, with the resultant effect of spurious lines or phase modulation (PM) spurs (spurious signals) in the output spectrum.
It has been suggested that phase errors due to PM spurs can be suppressed by dithering the phase at which the coarse-component accumulator is incremented in response to accumulation of the fine phase components.